This invention relates generally to monitoring systems having a continuously operating gas analyzer. More particularly, the present invention relates to area monitoring systems having xe2x80x9cdilutionxe2x80x9d source for supplying oxygen to a continuously operating combustible gas sensor.
The most common atmospheric danger in industry today is oxygen deficiency. Oxygen may be below the naturally occurring level of 20.9% by volume either due to consumption or displacement. Consumption sources include internal combustion engines and biological consumption sources, such as aerobic organisms. Displacement of oxygen can occur by the introduction of heavier-than-air gases into a confined space. Explosive gas dangers are the second most common hazard. Methane gas can result from the decomposition of organic matter in wastewater treatment plants and landfills. Natural gas leaks (methane) is a perpetual urban problem in sewers and subways. Propane gas can leak out of storage tanks or distribution piping.
Carbon monoxide and hydrogen sulfide are the two most commonly found toxic gases. Hydrogen sulfide is a byproduct of the decomposition of organic matter and can be present in significant concentrations in wastewater treatment plants and landfills. Hydrogen sulfide is also found in mining and oil fields, or wherever water comes in contact with elemental sulfur.
Federal regulations require that employers protect their workers from unsafe breathing atmospheres. The most common places where the atmosphere may not be safe are xe2x80x9cconfined spacesxe2x80x9d, for example wells, tanks, vessels, vaults, and unventilated rooms. Where access to such spaces is required and it is undesirable or impossible to spot test the atmosphere of such space prior to each entry, continuously operating area monitoring systems are frequently utilized. Other applications for area monitoring systems where unsafe atmospheres exist include steel mills, warehouses and parking garages (carbon monoxide danger), fertilizer manufacturing (ammonia danger), and plating operations (hydrogen cyanide).
Atmospheric sampling is conventionally accomplished by either a diffusion method or manual/continuous sample-draw. Diffusion allows for sampling the atmosphere only in the immediate area of the detector. Random air currents serve to deliver the detected gas to the sensor face. Sample-draw systems bring the gas sample from a remote location through tubing or pipes to the gas sensor. Manual sample-draw systems use a hand actuated aspirator bulb to pump the sample. Continuous sample-draw systems include the use of a battery-powered or mains-powered motorized sample draw pump.
A common application for industrial safety gas detection is the detection of explosive gases in reaction vessels that contain atmospheres largely comprised of nitrogen. Reaction vessels used for the refinery of petroleum products contain layers of catalyzing beds. These beds must be periodically replaced with new catalyzing material. The old catalyzing material is laden with volatile hydrocarbons that present an explosion risk. To eliminate the risk of explosion while workers are removing the old catalyst, the vessel is filled with nitrogen while the workers wear supplied air respirators or Self-Contained-Breathing-Apparatus. The atmosphere is still monitored for the presence of explosive gases in case the nitrogen purge is lost. Monitoring instrumentation is typically located outside the hazardous location and is monitored by dedicated personnel utilizing continuous sample-draw gas detection equipment.
The most common technology for monitoring for explosive gases are catalytic or xe2x80x9chot beadxe2x80x9d gas sensors. The sensors are constructed by coating tiny coils of platinum wire with a ceramic material, and then doping the coils or xe2x80x9cbeadsxe2x80x9d with a catalyst. In operation, sufficient current is directed through the sensor such that the surface temperature of the bead exceeds the temperature at which explosive gases will combust in the presence of oxygen and the catalyst. The temperature of the bead is elevated by heat released by the combustion of the explosive gas. The elevated temperature of the bead is reflected by the increased electrical resistance of the coil of platinum wire. Direct-reading instrumentation use this increased resistance to signal the presence of explosive gas. Hot bead sensors typically require about 10% oxygen concentration to operate.
Conventional monitoring systems utilize a xe2x80x9cdilution orificexe2x80x9d to combine fresh air with the sample stream to provide ample oxygen for the combustible gas sensor to operate. In practice, a fitting with an orifice open to the atmosphere is placed in the sample-draw tubing near the gas detector. By design, the orifice has a restriction to air flow about equal to the restriction provided by the length of tubing and any filtration that may be in the sampling system. Often times the orifice is adjustable so that the ratio of sample to dilution by fresh air is 1:1, causing the indicated readings to be halved. The operator must mentally multiply readings as indicated by the gas detector by two to arrive at the true sample readings.
The first of three problems with the use of the dilution orifice is that the dilution ratio can change by unknown amounts as the instrument is being used, thus affecting the indicated readings. The dilution ratio will change as the effective restriction of the sample filter changes as the sample filter becomes soiled. Subtle differences between the ambient pressure in the vicinity of the dilution orifice inlet and the ambient pressure in the vicinity of the sample tube inlet will also change the dilution ratio. Significant pressure changes can occur due to process requirements, inerting, and effects of wind.
The second problem with the use of a dilution orifice is that the user must remember whether the dilution orifice is in use and that the actual readings are twice the indicated readings. The user may forget that the dilution orifice is in use, as it may need to be removed from time to time to make straight un-diluted readings.
The third problem with the use of a dilution orifice is that daily calibrations of the gas measuring equipment must be made with the actual length of sample tube and the actual sample filtration system. This can be burdensome since it is often inconvenient to perform calibrations at the worksite and inconvenient to dismantle the sampling system and bring it along with the instrument to an office or laboratory for calibration.
Briefly stated, the invention in a preferred form is a monitoring system which comprises sample, dilution, and test subsystems. The sample subsystem includes a gas probe assembly defining a first end of a sample line and disposed in an area or adjacent to an object to be monitored. A sample pump mounted in the sample line provides a positive motive force for drawing a sample. A first sample pressure detector senses the pressure in the sample line intermediate the sample pump and the second end and provides a first sample pressure signal which is proportional to the sample flow rate. The dilution subsystem includes a dilution line having a first end vented to atmosphere. A dilution pump mounted in the dilution line provides a positive motive force for drawing the dilution air. A dilution pressure detector senses the pressure in the dilution line intermediate the dilution pump and the second end and provides a dilution pressure signal which is proportional to the dilution flow rate. The test subsystem includes a test line having a first end in fluid communication with the second ends of the sample and dilution lines. At least one gas sensor senses the presence of a gas in the test line and provides a gas signal proportional to the level of sensed gas in the test line.
Preferably, the sample subsystem also includes a second sample pressure detector for sensing the pressure in the sample line intermediate the first end and the sample pump. A particulate and hydrophobic filter mounted in the sample line intermediate the first end of the sample line and the sample pump removes moisture and particulate matter from the sample. A second particulate filter mounted in the sample line intermediate the first particulate filter and the sample pump provides redundant particulate removal capability. A flow orifice mounted in the sample line intermediate the sample pump and the second end of the sample line facilitates control of the sample flow rate.
Preferably, the dilution subsystem also includes a particulate and hydrophobic filter mounted in the dilution line intermediate the first end of the dilution line and the dilution pump. A flow orifice mounted in the dilution line intermediate the dilution pump and the second end of the dilution line facilitates control of the dilution flow rate.
The gas sensors of the test subsystem may include oxygen sensors, hydrocarbon sensors, infra-red absorption carbon dioxide sensors, and electrochemical toxic gas sensors.
A control subsystem of the monitoring system includes a controller having inputs in electrical communication with the sample pressure detectors, the dilution pressure detector, and the gas sensors. A software program stored in memory within the controller computes a dilution flow rate which is proportional to the dilution pressure signal and a sample signal which is proportional to the gas signal adjusted for the dilution flow rate, such that the sample signal is representative of the level of the sensed gas at the gas probe assembly. This sample signal may be transmitted to a local or remote display of the control subsystem.
The controller may also have an output in electrical communication with the start/stop control of the dilution pump and inputs in electrical communication with dilution pump and sample pump run sensors.
It is an object of the invention to provide a new and improved gas monitoring system
It is also an object of the invention to provide a monitoring system which provides a warning signal when the oxygen content in the vicinity of the monitoring system controls drops below a predetermined value.
It is further an object of the invention to provide a monitoring system which provides an indication of the level of the monitored gas where the value of the indication is continuously and automatically adjusted to account for the dilution air flow.
Other objects and advantages of the invention will become apparent from the drawings and specification.